1. Field of the Invention
The invention relates to a magnetic resonance apparatus comprising a superconductive magnet system for generating a steady magnetic field in a measuring space and to a magnet system for such an apparatus.
2. Prior Art
A superconductive magnetic resonance apparatus is disclosed in EP 138270 corresponding to commonly owned U.S. Application Ser. No. 657,636, filed Oct. 4, 1984. In this specification an apparatus is described having two coil systems which are arranged coaxially in the radial direction for generating a homogeneous interference-free magnetic field in a measuring space with stray field compensation by activation of the outer coil system.
For generating strong stationary magnetic fields in mr-apparatus, use is preferably made of superconductive magnetic coils, in particular when high requirements are imposed, for example, on the stability of the field to be generated. The superconductive coil customarily consists of a solenoid or a number of coaxial coils. The magnetic field is then directed along the axis of a cylinder formed by the coils and is usually rotationally symmetrical.
For reaching a very constant field a persistent mode is usually utilized in which the terminals of such a magnetic coil are short-circuited after a desired electric current through the coil is established by a power supply source. In this persistent mode the electric current through the coil remains substantially constant; only a small residual resistance of the coil can lead to a gradual decrease of current in time. The shape of the field of such a coil is mainly determined by the shape of the coil system. For correcting for deviations of the field from the shape in view as a result of inevitable manufacturing tolerances in the coil, use is generally made of superconductive or non-superconductive correction coils or of pieces of a magnetic material.
Such coils are usually wound from a wire in which the superconductive material is an alloy of the element niobium. Since the superconductive transition temperature of the said materials generally is well below 25K, such magnets are usually cooled with liquid helium or helium gas which is cooled by means of a cooling machine to a temperature of, for example, a few degrees above absolute zero.
In a magnet coil for generating a comparatively strong field, for example, at least 1 tesla, in a sufficiently large measuring space a large number of ampere turns of this wire are necessary and interruption somewhere in the wire may cause complete disturbance of the operation.
In addition to the above-mentioned niobium alloys a class of superconductive materials has recently become known which will briefly be referred to hereinafter as "ceramic superconductors". These materials are characterized in particular by a transition temperature which is significantly higher than 25K and a specific heat which is comparatively high also in the superconductive state.
Such materials can be brought in the superconducting state with a minimum of cooling means, as a result of which both construction and exploitation of a large superconductive magnet could become easier and cheaper. Unfortunately, the ceramic superconductors also have a few properties which makes them seem less suitable for use in a superconductive magnet system. The maximum permissible current density at which these materials are still reliably superconductive is often restricted. As a result of this, a large conductor cross-sectional area is desired for a large magnet. Furthermore, the ceramic superconductors are brittle and hence difficult to process to wire. In particular in magnets in which the stability of the field is important, said brittleness is a problem because any interruption in a desired superconductive short-circuited circuit will lead to an increase of the electric resistance and hence to a decrease of the current intensity and possibly to local heating which may rapidly expand by a further transition from the superconductive state.